Conventionally known body structures for rolling stock include a stainless steel body structure (body structure made of stainless steel for rolling stock), a light weight stainless steel body structure 201 (outside sheathing+framework+outside sheathing reinforcement members) as shown in FIG. 45(a), a double-sheet stainless steel body structure 202 (outside sheathing+integral press-molded inside sheathing) as shown in FIG. 45(b), and a double-skin type stainless steel body structure (see Japanese Patent Publication No. 2763983 for example). Such stainless steel body structures have many advantages including freedom from coating, easy maintenance, and anticorrosion.
In joining the outside sheathing and the outside sheathing reinforcement members together to form a body structure for rolling stock, resistance spot welding is frequently used from the viewpoint of reduction in thermal strain. To avoid shunting electric current to previously welded spots, the welding pitch is usually adjusted to about 50 to about 80 mm.
A body structure for general rolling stock, particularly, a side construction has some points to which attention should be paid in strength design. The “side construction”, as used here, means a structure comprising a single or plural side outside sheathing panels (each having outside sheathing and outside sheathing reinforcement members).
A side outside sheathing panel 101 is subjected mainly to an in-plane shear action by a vertical load F1 imposed by the car's own weight and passengers, as shown in FIG. 46(a). Also, the side outside sheathing panel is subjected to in-plane axial compressing and flexing actions by a load F2 imposed by forward and backward force exerted between adjacent cars (end compressing load) via a car coupler, as shown in FIG. 46(b). A breaking mode to which attention should be paid first in strength design is buckling of the side outside sheathing panel, and a basic structure is determined based on the criteria of such buckling.
For example, in a portion where the outside sheathing is extensively subjected to a compressing action (for example, a lower portion of a wainscot panel located centrally of a car under the end compressing load), outside sheathing reinforcement members 101 (stiffener) having required antiplane stiffness are joined to the inside surface of outside sheathing 102, as shown in FIG. 46(c). Since the side construction for rolling stock, in general, is subjected to a compressing action working in the longitudinal direction of the car more intensively than in any other direction, it is a common practice to position the outside sheathing reinforcement members (stiffener) on the inside surface of the outside sheathing to extend in the longitudinal direction of the car.
In a portion where the outside sheathing is extensively subjected mainly to shearing (for example, a door pocket part immediately above a bogie under vertical load), it is ideal that the outside sheathing reinforcement members are joined to the outside sheathing at an angle of 45° with respect to the rail direction. However, since such angled joining is complicated in the manufacture, the reinforcement members are actually positioned horizontally (in the rail direction) or vertically. These two positions are comparable to each other in terms of buckling strength.
However, such stainless steel body structures as mentioned above have several problems.
(i) A first problem is a lowered strength against general buckling and local buckling.
Resistance spot welding is frequently used to join the outside sheathing and the outside sheathing reinforcement members together from the viewpoint of reduction in thermal strain, as described above. To avoid shunting electric current to previously welded spots, the welding pitch is usually adjusted to about 50 to about 80 mm. In this case, it is possible that stress is not decentralized over each reinforcement member as desired and, hence, a theoretical buckling strength cannot be obtained.
Specifically, the stiffener panel may have an antiplane flexural stiffness lower than the theoretical value, which results in the possibility of occurrence of general buckling caused by a load that is lower than estimated. Also, the outside sheathing might buckle between adjacent welded spots by compression in a direction parallel with the outside sheathing reinforcement members (stiffeners). Thus, the buckling strength of the outside sheathing against such local buckling is also lower than the theoretical buckling strength.
As can be understood from the idea about buckling strength to be described later, with compression stress exerted on the outside sheathing in a direction parallel with outside sheathing reinforcement members (stiffeners) joined thereto at a pitch of 80 mm for example, the outside sheathing can withstand as low as about 60 Mpa if the reinforcement members are spot-welded to the outside sheathing at 80 mm pitch, though the outside sheathing can withstand a stress up to 160 MPa if the reinforcement members are continuously joined thereto.
Further, initial strain occurs on the outside sheathing due to strain about each spot caused by pressure contact. Such initial strain also causes local buckling strength to lower largely.
(ii) A second problem is permanent deformation in a stress concentrated portion (on the tensioned side) or local buckling (on the compressed side).
In the side outside sheathing, stress is concentrated at the corners of an opening portion of the side outside sheathing. A side construction for commuter cars, in particular, has many openings such as windows and doorways, and stress concentration at the corners of such an opening portion is problematic.
Such stress concentrated portions allow permanent deformation and buckling to occur on the tensioned side and the compressed side, respectively, which will finally lead to failure. A conceivable remedy for this problem is to increase the plate thickness on the tensioned side by providing the outside sheathing with an additional plate-shaped outside sheathing reinforcement member interiorly, thereby relieving the stress. Theoretically, the same remedy is possible on the compressed side. However, the conventional stainless steel body structure assembled by resistance spot welding involves some problems.
That is, in resistance spot welding, the welding pitch is usually adjusted to about 50 to about 80 mm, as described above. In this case, it is possible that stress is not decentralized over the reinforcement plate as desired and, hence, a theoretical buckling strength is not obtained. Further, even though the reinforcement plate is added, the number of welded spots is increased for joining of the reinforcement plate and, as a result, initial strain occurs in the outside sheathing due to strain about each spot caused by pressure contact and heating. The provision of such a reinforcement plate may cause local buckling strength to lower on the contrary.
(iii) A third problem is associated with watertightness.
Since resistance spot welding, which is frequently used in assembling a stainless steel body structure, can form nothing but a lap joint, joining of an outside sheathing to another outside sheathing or an edging member (a window frame, door mask or the like) is achieved by lap joint.
A contrivance to maintain the watertightness of such a joint is needed to prevent penetration of water from the outside. The watertightness of the lap joint is ensured by sandwiching a sealant between lap portions in welding because the lap portions define very fine clearances therebetween and because spot welding is an intermittent welding method. Alternatively, the watertightness is ensured by filling a sealant on lap end portions like a fillet.
However, it is possible that the seal is broken by aged deterioration of the sealant due to weather and washing to allow water to penetrate into the car. Note that such a problem will not occur with body structures of plain steel or aluminum alloy because such a body structure allows continuous welding to be used.
(vi) A fourth problem is associated with the aesthetic value of outside sheathing (side outside sheathing and end outside sheathing).
Since resistance spot welding, which is frequently used in assembling a stainless steel body structure, includes pressing in a spotted fashion during welding, strain occurs around each of the resulting welded spots due to pressing force and heating, while impressions are formed on the welded spots. The outside sheathing is aesthetically impaired by such strain and impressions. Impairment of the aesthetic value of the side outside sheathing or end outside sheathing, in particular, will lower the product value.
Though “scorch” on the outside sheathing resulting from spot welding can be eliminated by an electrolytic process, impressions are relatively deep and hence cannot be rendered invisible by polishing or a like process following the joining.
Though impressions can be covered with a color band (film), the impressions can be more conspicuous at some view angle.
(v) A fifth problem is the complexity of the inside framework.
Conventionally, as a structure for mounting interior trim or equipment on a body structure, screw seats are welded to the main structure or the inside framework, or fixtures are separately provided.
Such fixtures and screw seats are mostly designed individually for each car and a mounting place for such a fixture or screw seat differs depending on the car type and the part.
Accordingly, the count of parts including screw seats, inside framework and fixtures increases and, hence, very much man-hour is required for making and welding of such parts. In addition, dimensional control for mounting of such parts is complicated because mounting places are not standardized.
The inventors of the present invention have found that the aforementioned problems can be solved if laser welding is utilised instead of resistance spot welding in joining the outside sheathing and the outside sheathing reinforcement members together.
In using laser welding as described above, a certain penetration depth is needed to suppress variations in laser-welded joint strength in order to obtain a structure having stabilized quality. However, too large a penetration depth sometimes causes discoloration to occur on the reverse side opposite away from the welding side (back scorch) due to high temperature oxidation or allows the weld bead to be exposed. Methods of solving these problems are known (see Japanese Patent Publication No. 2929447 for example). Alternatively, it is possible to obviate oxidative discoloration by cooling the reverse side opposite away from the welding side during welding or by post-treatment.
With the conventional techniques, however, a member forming a lap joint is slightly bent at its edge due to contraction of the welded portion that has been heated locally by laser beam and such an edge bend may appear as a ridge-like weld mark along a weld line on the reverse side opposite away from the welded side. Such a ridge-like weld mark resulting from the edge bend is not so serious as compared to penetration through of a molten pool or oxidative discoloration caused by laser beam. However, the user requests that such a weld mark be eliminated in order to upgrade the quality and added value of the outside sheathing.
As a remedy for such a weld mark, it is possible to perform joining while controlling the laser power so as not to develop any weld mark. However, in cases where a 1.5 mm-thick lower plate and a 1.0 mm-thick upper plate are welded together, the bead width at the joint interface is 0.3 to 0.5 mm and the penetration depth into the lower plate is 0.1 to 0.2 mm. With such a penetration depth, joining cannot be achieved at all under the influence of some disturbance. Conversely, development of a ridge-like weld mark proves that the joint has been formed certainly. It is therefore difficult to ensure both the joint quality and the aesthetic quality of the outward appearance at a time.
In the case of a lower plate having a thickness of not less than 3 mm, it has been confirmed that little influence is exerted on the outside sheathing surface even if a sufficient penetration depth is ensured. However, an increase in the plate thickness of the lower plate more than necessary results in a considerable increase in the mass of the resulting structure undesirably.
As a result of repeated intensive study made by the inventors, it has been found that, if the outside surface of the outside sheathing is subjected to polishing (for example, belt grinder finishing generally employed for stainless steel body structures for rolling stock) substantially in parallel with the aforementioned weld line formed by laser beam, the weld line is rendered substantially invisible by scattering of light.